Ultra-high molecular weight polyethylene (UHMWPE), also known as high strength and high modulus polyethylene fibers, refers to high performance fibers prepared from polyethylene with a relative molecular weight of more than 1 million, through spinning, extracting, drying and ultra-stretching sequentially. Fiber reinforced composites prepared by use of the ultra-high molecular weight polyethylene fibers have the advantages of light weight, impact resistance, high dielectric property, etc., widely applied in aerospace field, sea area defense field, weapon equipment field and everyday industrial field.
In the prior art, the ultra-high molecular weight polyethylene fibers are generally prepared by use of gel spinning technique, created by DSM company in Netherlands firstly. In the gel spinning technique, polyethylene with a relative molecular weight of more than 1 million is commonly used as a raw material, the raw material is mixed with a suitable solvent, and swollen to obtain a suspension as a spinning dope, then the spinning dope is sheared, mixed uniformly and untwisted via a screw extruder, and extrusion-stretched via a spinning pack and condensed and formed to obtain gelatinized pre-oriented filaments, and then the gelatinized pre-oriented filaments are extracted, dried and ultra-stretched to obtain ultra-high molecular weight polyethylene fibers.
UHMWPE has high strength and high modulus performance, because after the UHMWPE powder is dissolved in a solvent, the entanglement among molecular chains is disentangled at some degree, the gelatinized pre-oriented filaments formed by extruding via a spinneret and flash cooling keep the disentangled state of molecular chains in the filaments; and then extracted and multistage ultra-hot-stretched to fully extend the PE macromolecular chain in the axial direction, so that the crystallinity and the degree of orientation are both improved correspondingly. At the same time, the chain-folded lamellae in the molecular structure are converted into extended chains, so as to obtain high-strength and high-modulus polyethylene fibers.
The thinner the UHMWPE single filament and the better the fiber mechanical performance, the thinner the single filament and the softer the hand feeling of the fabric prepared therefrom according to the Griffth formula. However, in the gel spinning process for UHMWPE fibers, it is necessary to perform higher-stretching for the gelatinized pre-oriented filaments, which has a very high demand for drawability and crystalline structure of the gelatinized pre-oriented filaments. The existing technique only can realize subsequent hot stretching with a factor of 30-40, when the stretching factor exceeds the above stretching factor, single- or multi-fiber breakage of the UHMWPE fiber bundle usually occurs.
The inventor has studied and found that the crystallization among molecular chains not only appears in hot stretching process. In fact, when the spinning feed solution is sheared in a screw and ejected through a spinneret and flash-cooled and formed to obtain freshly formed filaments, part of the disentangled macromolecular chains firstly form a shish-kebab center line part having extended chain crystal structure under the action of orientation, the shish-kebab center line part can be used as crystal nucleus to induce and generate a series of chain-folded lamellas, so as to constitute a shish-kebab structure. In this way, in the subsequent hot stretching process, after the folded chain of the molecular chain is gradually opened and stretched, lamella generates recrystallization in the destroyed process, and the orthorhombic system is partially converted into a more stable hexagonal crystal system, so as to obtain a molecular crystal structure in the form of orthorhombic system and hexagonal crystal system together. The inventor has further studied and found that the reason why the UHMWPE fiber bundle has occurred partial fiber breakage in the subsequent high-stretching process is that in the molecular crystal structure of the fibers, the arrangement of the orthorhombic system and the hexagonal crystal system is nonuniform, resulting in the inhomogeneity of fiber mechanical performance, and partial mechanical performance of the fiber is relatively poor, so that in the high-stretching process, fiber breakage easily occurs at the part with relatively weak mechanical performance, therefore it is difficult to realize relatively high-factor stretching.
Accordingly, the present invention contemplates the adjustment for gel spinning process to improve the uniformity of shish-kebab formation in gelatinized pre-oriented filaments, thus improving the arrangement uniformity of orthorhombic system and hexagonal crystal system formed after the subsequent hot stretching and recrystallization, and finally improving the homogeneity of fiber mechanical performance, so as to realize single filament fine denier production and achieve high strength and high modulus and excellent performance.